Generally, hot restartability of a small internal combustion engine provided with a diaphragm carburetor is not good for several reasons mentioned below:
(a) A metering chamber is heated by heat of the engine, atmosphere heat, radiant heat of sunshine and the like after the engine has been stopped. This occurs particularly after operation with high load under a burning sun in a summer season. If the metering chamber is heated as described above, fuel having a low boiling point interiorly stored is changed into vapor and flows from a fuel passage to the air inlet and a venturi portion of a carburetor. At the same time, liquid fuel must also flow out and vapor and liquid in the air inlet and venturi may flow into an engine crankcase depending on the attitude of the engine.
(b) Particularly, in the period 15 to 20 minutes or so after the engine has been stopped, the fuel in the metering chamber discharges completely into the air inlet, and the interior of the fuel chamber is filled with fuel vapor.
(c) When the recoil starter is pulled to restart the engine, fuel in the form of liquid and vapor in the air inlet and venturi portion is taken into the engine all at once and supplied in the form of a super-rich mixture. Therefore, the engine will not start.
Particularly, at the time of restarting the engine in 15 to 20 minutes after the engine has been stopped, the engine is still in a hot state requiring no rich-mixture, and therefore, when the super-rich mixture is supplied, the engine is more difficult to start.
(d) In such a state as described above, roping, that is, pulling the recoil starter rope, is carried out several times to discharge the super-rich mixture, and the initial explosion can be effected only when the interior of the cylinder has a mixture in the range of combustion.
(e) When a throttle valve is opened and roping is effected at a start position, a mixture may be exhausted with less roping to effect the initial explosion. However, since the throttle valve is opened, venturi pressure is so low as not to be able to draw vapor from the metering chamber, and even if the initial explosion is effected, the engine will not continue running but soon stops. Even if roping is effected over and over again thereafter, the engine will not start.
(f) In the case where the throttle valve is in the idling position, roping has to be done over and over again to exhaust the rich mixture. An ignition plug may become covered with the mixture depending on the attitude of the engine and the position of the ignition plug, and this also will contribute to the failure to restart.
(g) The outflow of fuel in the air inlet and venturi portion from the metering chamber after the engine has been stopped makes it difficult to provide hot restart regardless of whether the throttle valve portion is opened or in the idling position.
(h) When the choke valve is used in the state wherein the engine is hot, fuel accumulated in the air inlet is supplied in its richer state to the engine, and again the engine is difficult to restart.
As means for solving these problems noted above, the present applicant has proposed an improved diaphragm carburetor as described in U.S. patent application, Ser. No. 36,442, filed Apr. 9, 1987, and patent application Ser. No. 84,894, filed Aug. 13, 1987. According to the diaphragm carburetors disclosed in said patent applications, fuel is discharged from the metering chamber through an exhaust valve and directly discharged onto the ground, and sometimes splashed on the clothes of an operator, which presents the danger of fire and other inconvenience.
Furthermore, in the case of the chain saw, a carburetor is often mounted inside of a case containing the carburetor and air required for combustion is introduced into the carburetor through this case. Therefore, fuel discharged from the metering chamber is discharged into the case, and since the case is extremely hot, fuel immediately vaporizes to fill the interior of the case. Thus, a super-rich mixture is supplied when the engine is to be restarted making it difficult to smoothly start the engine.
In order to overcome the aforementioned problem, the present invention provides a diaphragm carburetor for the internal combustion engine wherein a metering chamber of a diaphragm carburetor is closed during operation of the engine while being opened to atmosphere when the engine is not running and fuel discharged from the metering chamber is temporarily stored in a liquid intake member.
For achieving the above-described object, the present invention provides an arrangement wherein a liquid absorbing intake member is connected externally of an atmospheric port of an exhaust valve for closing a metering chamber of the diaphragm carburetor during operation of the engine while opening said chamber to atmosphere when the engine is not running.